Let's take the breath as a simple example. Without learning how to control your breath, today's fast-paced exercise sequences may cause you to hold your breath when the going gets tough or to breath as often as 20 times per minute. At this pace, you can not refine the breath, link the breath to movement, or discover the effect of the breath on the mind. In Yoga, you should draw the breath slowly like drinking through a straw, not by sucking the air in, breathing maybe 4-6 times per minute even in demanding poses. This is quite difficult to achieve and requires some mastery over internal processes which can only be learned directly from a teacher.
This is why the old texts start out the journey of Yoga with various preparatory practices, e.g. Kriyas for gaining awareness of the inner body, and bandhas to regulate energetic processes. In today's Yoga classes, these practices are not utilized because they take some time to master and today's Yoga students want a "workout" when coming to class. If you want to go deeper with your practice and use Yoga as a tool for self-knowledge, you have to come back to these basics and learn the preparatory practices as they will entirely change your practice and focus during practice.
If you ask yourself what physical sensations you have in your body, then you may realize that there is very little awareness of the body unless something is wrong and your body wants you to pay attention. You feel the external body if you hurt yourself or maybe you feel tension in your shoulders and neck. Similarly, you feel the internal body if you are hungry or if you need to go to the bathroom. Looking more closely, you will find that the external body often holds a lot of tension when doing Yoga (gripping your toes, tension in the face and shoulders), while the internal body feels "mushy", soft without much feedback. In traditional Yoga, you strive for the opposite, a firm internal body giving plenty of feedback, while keeping the external body soft, relaxed, and responsive.
These concepts can not be learned if you are pushing, pumping and puffing in a physically challenging position. A knowledgeable teacher will impart these techniques in simple poses. Even simply standing can feel quite challenging if you maintain a slow, refined breath, and a focused, undivided mind.
Over time, you will learn to feel and interpret more and more subtle sensations of the body, and by learning to pay attention to them, you will become aware of the energetic principles present in your body. After years of practice, your Yoga practice becomes a string of slow breaths and each breath is linked to a specific movement. The mind remains focused within, the gaze is soft, all sense organs are used intentionally. At the end of such a practice, the body feels energized, not tired, and the mind feels quiet, not agitated. In traditional Yoga, good health and fitness are simply side-effects of this practice. The actual goal is self-realization, Rāja Yoga, the state of liberation. But that's worthy of another post in the future.
So is this worth it? This sounds like a lot of effort just to learn to breath slower and to move more mindfully. Nobody can answer this for you. Traditional Yoga is a method for lasting transformation, but it is not easily accomplished. There are many obstacles on this path and you need a strong desire to overcome them and a good teacher to guide you through. Most people, after some initial excitement, find ways to sabotage every attempt to true and lasting change. It is, of course, easier to keep things the way they are. However, this has little to do with the practice and more with how our minds work. My teacher says that if you want to find water, dig one hole and dig deep, don't dig in 20 places or you will never find water. When the digging gets hard, it is very tempting to start digging somewhere else. Patience and persistence are not qualities that come easy. But impatience and apathy lead to suffering, they keep us sleep-walking in a reactionary way of life. Traditional Yoga is a way, albeit a challenging one, to wake up and to remember who we are.